In a known manner, an electric motor can be implemented into an electric power steering system so that the motor provides assist torque for operator-responsive steering. Additionally, in some steering systems it is desirable to have the electric motor provide return-to-center assist torque to aid or optimize the return-to-center operation of the steering system and steering wheel. Example return-to-center operation occurs when the vehicle is traveling at road velocities and road forces on the steered wheels tend to return the steering system to the straight-ahead or on-center position, and do so when the operator input steering torque is removed.
Different modes of operating an electric power steering system provide different control challenges. For example, during high assist torque steering operation, a highly damped control operation is desirable to enhance closed loop stability of the system. However, during other steering operations, such as a return-to-center operation that occurs when a vehicle driver releases the hand wheel (steering wheel) while the vehicle is being driven and the wheels are steered in a turn, the highly damped control system results in less than optimal on-center feel and less than optimal return-to-center velocity.
Accounting for these variations in control response of the steering system may require complex control strategies, increasing the cost of the necessary controller and therefore increasing the cost of the system.